A typical LCD device includes an LCD panel, and a backlight module mounted under the LCD panel for supplying light beams thereto. The backlight module mainly includes a light source and a light guide plate. The light guide plate is generally made of a transparent acrylic plastic, and is used for guiding light beams emitted from the light source in order to uniformly illuminate the LCD panel.
In order to diffuse the light beams and emit them uniformly from a top surface of the light guide plate, it is common for protrusions or recesses to be formed at a bottom surface of the light guide plate. Alternatively, a pattern of light diffusion dots may be formed on the bottom surface of the light guide plate.
Referring to FIG. 7, a conventional backlight module 20 includes a light guide plate 21, a light source 22, and a reflective sheet 23. The light guide plate 21 includes a light incident surface 211, a light output surface 213 adjacent to the light incident surface 211, and a bottom surface 212 opposite to the light output surface 213. The bottom surface 212 includes a plurality of parallel, regularly arranged V-shaped prisms 214. The light source 22 is disposed adjacent to the light incident surface 211, and the reflective sheet 23 is disposed adjacent to the bottom surface 212.
Referring to FIG. 8, an exemplary optical path of a light beam transmitting in the backlight module 20 is shown. Light beams emitted by the light source 22 propagate within the light guide plate 21, are reflected and refracted by the light guide plate 21 and the reflective sheet 23, and exit from the output surface 213 to illuminate an associated liquid crystal display panel.
Also referring to FIG. 9, this shows a light intensity distribution graph of light beams output from the light guide plate 21. In the graph, X, Y coordinate values are plotted. An angle between light beams output from the output surface 213 and a line normal to the output surface 213 is taken as a value of the X-coordinate, and a relative intensity of the light beams is taken as a value of the Y-coordinate. The curve A1 denotes a relative intensity of the light beams in a first plane perpendicular to both the light output surface 213 and the light incident surface 211. The curve B1 denotes a relative intensity of the light beams in a second plane that maintains an angle of 45° relative to the first plane. The curve C1 denotes a relative intensity of the light beams in a third plane that maintains an angle of 90° relative to the first plane. The curve D1 denotes a relative intensity of the light beams in a fourth plane that maintains an angle of 135° relative to the first plane. According to FIG. 9, the V-shaped prisms 214 at the bottom surface 212 tend to concentrate the emitting angles of the output light beams.
The intensity of the output light beams located in the range of ±20° in each of the first, second, and fourth planes is greatest, and approaches a maximum value of 7. On the other hand, the intensity of the output light beams located in the range of ±20° in the third plane is low. Even the maximum value achieved at the angle of 0° is low—i.e. approximately 2 only. That is, with the configuration of the V-shaped prisms 214, the output light beams cannot be satisfactorily concentrated in the third plane that is perpendicular to the light output surface 213 and parallel to the light incident surface 211. In other words, the overall intensity distribution of the output light beams is not even.
Accordingly, what is needed is a light guide plate and a backlight module that can overcome the above-described deficiencies.